1. Technical Field of the Invention
The present invention relates to the construction of structures below ground by means of a technique known as “top-down” construction. In particular, the present invention relates to methods and apparatus for placing and positioning an element, such as a steel section or stanchion, within a pile shaft. Subsequently the element, which is generally positioned vertically above a load bearing pile, may be used to support loads from an above ground structure and to transmit load from ground level to the head of a load bearing pile.
2. Prior Art
Briefly, top-down construction involves the following steps:
i) construction of a perimeter wall which is installed from the prevailing ground level;
ii) installation of load-bearing piles within the curtilage of the perimeter wall. To construct each load bearing pile, a pile shaft of required depth is excavated and may be partially filled with concrete or grout. If the shaft is partially filled at this stage, the concrete of the pile is terminated at a distance below ground that generally corresponds to the level of the (future) basement slab;
iii) insertion and positioning of an element, which may be a steel column, hollow steel section, precast concrete, or any suitable member, into the ‘empty’ bore of the pile shaft, above the level of the concrete or “pile head”. The element is designed to transmit load from ground level to the pile head. Generally the requirement is for the element to be positioned vertically, although other orientation requirements may be required for specific applications;
iv) Once the element has been positioned within the bore, it is either lowered onto the top of the set concrete in the pile, so that load transmission between the element and the pile is by end-bearing onto the concrete head, or it is plunged some distance into the concrete to become embedded in the pile before it sets.
As an alternative to the above method, it is possible for the element to be positioned within the excavated bore shaft before any concrete is placed. Once the element has been positioned within the shaft, concrete is placed to the base of the excavated shaft by means of a concrete supply pipe, and the bore filled to the required depth.
Several methods have been used in the past for positioning an element in a pile shaft during top-down construction techniques. For example, during the construction of a cast in place pile, a shaft lining tube of steel or reinforced concrete was installed and the bore was then partially filled with concrete. Once the concrete had set it was necessary for an operative to be lowered into the casing in order to clean the concrete surface and to fix a base plate or similar device to the head of the concrete. The element was then lowered into position and fixed to the base plate by the operative using surveying techniques to adjust the plan position of the element. Generally, the casing which was installed to ensure safety during the manned descent was left in place. The cost of the casings and the safety measures taken meant that the costs involved in the operation were substantial.
More recently, a variety of tools have been developed which allow the element to be positioned remotely, i.e. from ground level, by means of mechanical, electrical or hydraulic drive means.
When positioning an element within a bore hole it is important to be able to monitor and/or adjust the position of the element at or close to ground level, and also to have a knowledge of the position of the element at a lower level in the pile shaft, preferably at or near the head of the (future) concrete pile.
In EP 0302707, an apparatus for positioning a column in an underground pile shaft is described which comprises a locating frame having at least one pair of adjustable guide frames at either end of the frame. The frame is placed into a borehole so as to define an interior space through which the element is lowered, and is braced against the inner wall of a temporary casing which lines the pile shaft. The guide frames are remotely adjustable and can be operated to locate the column at that level. Once the guide frame at the upper level has been adjusted, the guide frame at the lower level is also adjusted so as to adjust the plan position of the column at the lower level.
There are a number of problems which have been encountered with the locating device described in EP 0302707. Firstly, the frames are inconvenient to store or handle due to their weight and size. Furthermore, the frame requires a temporary casing to be inserted into the bore to a depth of at least the length of the frame. The temporary casing is cumbersome and the process of installing it often time consuming.